1. Field of the Invention
The present invention relates to ceramics that thermally expands little containing cordierite as a main crystal phase. In particular, the invention relates to ceramics that thermally expands little and is adapted for use in various devices used for a process for producing semiconductors, such as a semiconductor wafer support fitting like a vacuum chuck, succeptor, electrostatic chuck, or a stage or a member for supporting an optical element in a lithography apparatus.
2. Description of the Prior Art
The cordierite-type sintered product has heretofore been known as ceramics that thermally expands little, and has been used for filters, honeycombs and refractories. The cordierite-type sintered product is obtained by using a cordierite powder or a powder in which is mixed MgO, Al.sub.2 Do.sub.3 and SiO.sub.2 in amounts capable of forming cordierite, by adding, to this powder, a sintering assistant such as an oxide of a rare earth element, SiO.sub.2, CaO or MgO, molding the mixture into a predetermined shape, and firing the obtained molded article at 1000 to 1400.degree. C. (Japanese Examined patent Publication (Kokoku) No. 3629/1982 and Japanese Unexamined Patent Publication (Kokai) No. 229760/1990).
Various members used for the process for producing semiconductors such as LSIs, e.g., semiconductor wafer support fittings such as vacuum chuck, succeptor, electrostatic chuck, and a stage and members for supporting an optical element is a lithography apparatus, have heretofore been produced by using ceramics such as alumina or silicon nitride on account of the reason that it is chemically stable and is obtained at a reduced cost. Accompanying a trend toward a high integration degree in the LSIs in recent years, however, high resolution circuits have been formed in the semiconductor wafer requiring high degree of precision. For example, the lines of the circuits have a width of the order of submicrons. In a lithography apparatus used for forming the circuits of this kind, the positioning precision required for the stage for holding the wafer in which the circuit is to be formed must be 100 nm or smaller. The ceramics such as alumina and silicon nitride have considerably large coefficients of thermal expansion at 10 to 40.degree. C. (5.2.times.10.sup.-6 /.degree. C. in the case of alumina, and 1.5.times.10.sup.-6 /.degree. C. in the case of silicon nitride). With such ceramics, a change of 0.1.degree. C. in the temperature of the atmosphere results in the deformation of about several hundred nanometers, making it no longer possible to satisfy the above-mentioned requirement of precision.
It has also been proposed already to apply the cordierite-type sintered product to various parts used for a process for producing semiconductors (Japanese Unexamined Patent Publication (Kokai) No. 191422/1989, Japanese Examined Patent Publication (Kokoku) No. 97675/1994). The cordierite-type sintered product thermally expands less than the above-mentioned alumina or silicon nitride, and is favorable form the standpoint of preventing a drop in the precision of the circuit caused by thermal expansion. This sintered product, however, has low rigidity which is a defect. That is, the semiconductor wafer support member such as a stage in the lithography apparatus moves at a high speed to a region where the exposure to light is to be executed, stops at a predetermined position and , then, the wafer placed on the support member is exposed to light. The support member made of the cordierite-type sintered product having a low rigidity develops vibration when it has stopped moving, and the exposure to light is executed in a vibrating state, resulting in a drip in the precision of exposure to a conspicuous degree. The drop in the precision of exposure becomes conspicuous as the lines of the circuit formed by exposure to light become fine, casting a fatal problem from the standpoint of forming high resolution circuits.
Moreover, the members supporting the optical elements in the lithography apparatus transmits vibration to the optical elements accompanying the motion of the stage. When the exposure is effected relying upon such optical elements, therefore, the light beam vibrates causing the focal point to be blurred or deviated and, eventually, causing the precision of exposure to be greatly deteriorated.